Sprocket clutch

ABSTRACT

A clutch assembly for an accumulating roller conveyer such that torque transmission is accomplished by using clutch plates of deformable material which in association engage with the conveyer drive and conveyer rollers to allow the bearings to deform the surfaces of the clutch plates in transmitting the torque from the drive to the rollers.

This invention relates to a slipping sprocket clutch for an accumulatingroller conveyor.

An accumulating roller conveyor is a conveyor which has a plurality oftransverse driven rollers, each of the rollers being driven via arespective slipping clutch. Each of the rollers is mounted on arespective shaft for conjoint rotation therewith, the shafts beingdrivable, via the slipping clutches, by a drive member extendinglongitudinally along the conveyor at one side thereof. The clutches aredesigned to permit slipping between the rollers and the drive member sothat, when an item being conveyed is stopped, the clutches associatedwith the rollers engaging the item will slip. This ensures that theserollers are stationary, whilst all the other rollers continue to bedriven Any other item on the conveyor does, therefore, continue to beconveyed. Consequently, items upstream of the stopped item will tend toaccumulate behind the stopped item, and this gives rise to the termaccumulating roller conveyor.

A known type of accumulating roller conveyor has a rotatable drive shaftextending along one side of the conveyor. Drive is transmitted to rollershafts via O-rings, each of which passes round the drive shaft andengages with a groove in a respective roller shaft. The O-ringsconstitute slipping clutches as they slip in the associated grooves whenthe corresponding rollers are prevented from rotating by a stationaryitem in contact therewith. The disadvantage of this type of conveyor isthat the torque transmission from the drive shaft to the roller shaftsdepends upon the tension in the O-rings, and this is often insufficientto provide adequate torque transmission. Moreover, it is extremelydifficult to control the tension in the O-rings so as to provide giventorque transmission properties.

Another known type of accumulating roller conveyor has an endless drivechain constituting the drive member. This drive chain drives sprocketswhich in turn drive the roller shafts via slipping clutches. Each ofthese slipping clutches is a complicated ratchet arrangement having aplurality of cams and spring-loaded balls. The disadvantage of this typeof conveyor is that it is difficult to assemble the clutches so as togive a desired torque transmission, and it is even more difficultsubsequently to change their torque transmission properties. Because ofits complicated construction, this type of conveyor is also relativelyexpensive.

In another known type of accumulating roller conveyor, the roller driveshafts are hexagonal, and the slipping clutches are constituted bydeformable plastics tubes held in pockets in cages surrounding theshafts. The deformable plastics tubes associated with each of thehexagonal shafts are drivably engageable with the internal cylindricalsurface of the corresponding roller so as to transmit torque thereto.Here again, drive is transmitted to the roller drive shafts, viasprockets fixed thereto, from an endless drive chain. If a given rolleris prevented from rotation by a stationary item in contact therewith,the respective roller drive shaft can continue to rotate, with theplastics tubes being successively more and less deformed between thatshaft and the internal cylindrical surface of the respective roller asthe pointed regions of the shaft engage the plastics tubes. Thedisadvantage of this arrangement is that each clutch assembly needs fourbearings, so that it is relatively expensive. Moreover, the rollersthemselves require accurate machining, and extra sleeves are needed toaccommodate the hexagonal shafts, and these steps entail furtherexpense.

U.S. patent specification No. 4,286,441 describes an automatic slipcoupling assembly for a roller conveyor. The slip coupling assemblyincorporates a ball bearing which permits an associated sprocket tocontinue to rotate with slip taking place between the balls and anassociated clutch plate, thereby preventing damage to conveyed goods ifa jam occurs. Unfortunately, the bearing assembly is mounted on ajournal bearing that can take up only radial loads. Any axial loads aretaken up by the associated conveyor roller, and this leads to excessivewear. Another disadvantage of this arrangement is that many of the partsthereof are made of metal which cannot deform to absorb energy as theclutch slips. This again results in forces being applied to the movingparts of the assembly, and increases the rate of wear.

Known accumulating roller conveyor clutches also suffer from thedisadvantage of a relatively low (0.2 Nm) torque transmissioncapability.

The aim of the invention is to provide a slipping sprocket clutch for anaccumulating roller conveyor which is cheaper, more reliable, and hasimproved torque transmission characteristics when compared with knownclutches.

The present invention provides a clutch assembly for an accumulatingroller conveyor, the clutch assembly comprising a drive member formed bya sprocket which is drivably engageable with a conveyor drive means, adriven member which is drivably engageable with a roller of theconveyor, a first clutch plate associated with the drive member, asecond clutch plate associated with the driven member, a bearingpositioned between the first and second clutch plates and having arolling element race, and thrust means for forcing the first and secondclutch plates towards one another and against the rolling elements ofthe bearing, wherein the first and second clutch plates are made of anelastically-deformable plastics material, and wherein the drive memberis rotatably mounted on a shaft sleeve by means of an angular contactbearing within the sprocket, the first and second clutch plates beinggenerally annular and surrounding the shaft sleeve, and the shaft sleevebeing fixed to, and rotatable with, the driven member, and thearrangement is such that the rolling elements elastically deform thesurfaces of the first and second clutch plates whereby torque can betransmitted from the first clutch plate to the second clutch plate.

Advantageously, the sprocket is drivably engageable with an endlessdrive chain constituting the conveyor drive means, and the first clutchplate is constituted by a surface of the sprocket. Preferably, thearrangement is such that the rolling elements of the angular contactbearing elastically deform the races associated therewith.

In a preferred embodiment, the thrust means may be a thrust nut which isengageable with an externally-threaded portion of the shaft sleeve.Conveniently, the sprocket is made of a low-friction, thermoplasticmaterial such as acetal.

The driven member may be a drive shaft fixed to, and rotatable with, theroller. Advantageously, the second clutch plate is constituted by asurface of a retaining ring, the retaining ring being positioned betweenthe thrust means and the bearing. Conveniently, the retaining ring ismade of a low-friction, thermoplastic material such as acetal. The shaftsleeve may also be made of a low-friction, thermoplastic material suchas acetal. Preferably, the rolling elements of the bearing are ballsmade of a self-cleaning, high chromium special steel.

The invention also provides an accumulating roller conveyor comprising aplurality of rollers, conveyor drive means for driving the rollers, anda respective clutch assembly between the conveyor drive means and eachof the rollers, wherein each clutch assembly is as defined above.

Preferably, the conveyor drive means is an endless drive chain indrivable engagement with the sprockets of the clutch assemblies.

An accumulating roller conveyor incorporating slipping sprocket clutchesconstructed in accordance with the invention will now be described indetail, by way of example, with reference to the accompanying drawings,in which:

FIG. 1 is a side elevation of part of the conveyor;

FIG. 2 is an end elevation of the conveyor; and

FIG. 3 is a schematic sectional elevation of a slipping sprocket clutchof the conveyor.

Referring to the drawings, FIGS. 1 and 2 show part of one side plate 1of an accumulating roller conveyor. The side plate 1 supports respectivefirst ends of a plurality of transverse rollers 2, the other ends of therollers being supported by a similar side plate (not shown). Each roller2 is fixed to a respective shaft 3 for conjoint rotation therewith, andthe ends of the shafts 3 are rotatably supported in the side plates byroller bearings 4 (one of which is shown in FIG. 2). The rollers 2 arehollow tubes made of stainless steel, and they are fixed to their shafts3 by means of end plates 2a and grub screws 2b. The end plates 2a aremade of aluminum.

The shafts 3 are rotatably driven, via respective slipping sprocketclutches (one of which is indicated generally by the reference numeral5), by an endless drive chain 6. Each of the clutches 5 includes a shaftsleeve 7 (see FIG. 3), which is fixed to its associated shaft 3 by meansof a split pin 3a which passes through aligned holes in the shaft andthe shaft sleeve. A sprocket 8 is rotatably mounted on the shaft sleeve7 via an angular contact bearing assembly 9, the balls 9a of which runin a groove 7a in the shaft sleeve. The balls 9a are held and spaced bya cage 9b. A thrust ring 10 supporting a plurality of ball bearings 10abears against the rear face of the sprocket 8, and the thrust ring isheld against this rear face by a retaining ring 11 and a thrust nut 12,the thrust nut being screwed onto an externally-threaded portion 7b ofthe shaft sleeve 7. A lock nut 13 is provided for holding the thrust nut12 in position.

The shaft sleeve 7, the sprocket 8, the ring of the bearing assembly 9,the thrust ring 10 and the retaining ring 11 are all made ofinjection-moulded acetal, the threaded portion 7b of the shaft sleevebeing formed during the injection moulding process. Alternatively, theseparts could be made of polypropylene or other suitable low-friction,oil-free thermoplastic material. The balls 9a and 10a are made of a highchromium special steel that has self-cleaning properties. The nuts 12and 13 are made of steel.

In use torque is transmitted from the drive chain 6 to each of theshafts 3 via four surfaces of the associated clutch, namely:

(i) the surface between the sprocket 8 and the balls 9a of the angularcontact bearing assembly 9;

(ii) the surface between the balls 9a and the groove 7a of the shaftsleeve 7;

(iii) the surface between the sprocket 8 and the balls 10a of the thrustring 10; and

(iv) the surface between the balls 10a of the thrust ring 10 and theretaining ring 11.

The amount of torque transmitted is dependent upon the degree to whichthe thrust nut 12 is tightened, and ranges from 0 to 0.4 Nm for shaftrotation speeds of from 0 to 80 r.p.m. Moreover, it is a relatively easytask to alter the torque transmission characteristics of the clutches 5,after installation, by merely adjusting the degree to which the thrustnuts 12 are tightened.

When a given roller 2 is prevented from rotation by engagement with astationary item, the associated clutch 5 slips, this slip occurring atall four surfaces mentioned above. The slip between the balls 10a andthe surfaces of the sprocket 8 and the retaining ring 11 is subject tothe Brinnel effect. This effect is a result of the elastic deformationof the plastic material of these surfaces by the balls 10a. Thus, whentorque is being transmitted and no slip is occurring, the compressiveforce applied by the thrust nut 12 causes the balls 10a to embedthemselves slightly into the opposing planar surfaces of the sprocket 8and the retaining ring 11, the plastic material of these parts deformingelastically to permit this. This inward elastic deformation toaccommodate the balls 10a causes a similar outwards elastic deformationto form `peaks` either side of each ball. Once slip occurs, the balls10a begin to rotate and to move, around a circular path, relative toboth these surfaces. As this happens, the `peaks` in front of the balls10a are pushed round with the balls, so that the two surfaces aresubjected to a continuous wave-like elastic deformation as the clutch 5begins to slip.

Obviously, torque continues to be transmitted from the sprocket 8 to theshaft 3 of a given clutch as it slips, this torque being transmitted tothe associated roller 2 which continues to try and advance thestationary item in contact therewith. The Brinnel effect is at a maximumwhen a given clutch 5 begins to slip, and hence more torque istransmitted at this time. This is because the previous lack of relativemovement between the balls 10a and the two engaging surfaces has led torelatively deep depressions being formed in these surfaces (and hence tothe formation of relatively large `peaks` on either side of each ball),these relatively large `peaks` helping to transmit a relatively hightorque. As slip continues, the `peaks` and depressions become lesspronounced, so torque transmission is reduced (to about 80% of theinitial slipping value).

Similarly, slip occurs between the balls 9a of the angular contactbearing assembly 9 and the associated races defined by the shaft sleeve7 and the sprocket 8, and this slip is also subject to the Brinneleffect, and so the effect described above is enhanced.

This reduction in torque transmission, which results from a reduction inthe Brinnel effect, has the advantage of reducing what is known as the`queuing` force. Thus, when one item stops on the conveyor, thefollowing items stop one after another in a queue behind the firststopped item. As the clutches 5 associated with the rollers 2 preventedfrom rotating by a stationary item begin to slip, the torque transmittedis reduced from what is known as the driving force to that of the`queuing` force (which as explained above is about 80% of the drivingforce). Consequently, the force which tries to force the items againstone another is reduced, and this is important where the items arefragile or squashable. Another advantage of this reduced `queuing` forceis that it leads to a reduction in power consumption during queuing.

The angular contact bearing assembly 9 has the advantage of allowingboth the radial and the longitudinal forces, applied by the chain 6 andthe thrust nut 12 respectively, to be taken up within the clutchassembly.

Because the sprocket 8 and the shaft sleeve 7 (which define the races ofthe angular contact bearing assembly 9) are made of plastics material,as the clutch slips energy can be absorbed by the clutch, not only atthe bearing surfaces, but also as a result of the deflection of thesprocket and the shaft sleeve themselves. This deflection mainly takesplace radially about the longitudinal axis of the sprocket 8, thesprocket increasing in size diametrically and cyclically as the balls 9arotate within the bearing. This is due to the surfaces of the angularcontact bearing assembly 9 being Brinnelled, giving variations in thedistance between the two bearing surfaces of the bearing.

As shown in FIG. 1, the working run of the drive chain 6 (that is to saythat portion of the drive chain engaging the sprockets 8) is supportedby a shelf 1a fixed to the adjacent side plate 1. The upper surface ofthis shelf 1a is provided with a wear-resistant covering to minimise thewear on the drive chain 6.

The type of clutch 5 described above has a number of advantages comparedwith known accumulating conveyor clutches. In particular, the clutch 5is relatively cheap to manufacture, and is easy to assemble. Moreover,this type of clutch is easily adjustable to vary the amount of torquetransmitted. Furthermore, because torque is transmitted as a result ofthe Brinnel effect, a relatively high torque transmission capabilityresults.

I claim:
 1. A clutch assembly for an accumulating roller conveyor, theclutch assembly comprising a drive member formed by a sprocket which isdrivably engageable with a conveyor drive means, a driven member whichis drivably engageable with a roller of the conveyor, a first clutchplate associated with the drive member, a second clutch plate associatedwith the driven member, a bearing positioned between the first andsecond clutch plates and having a rolling element race, and thrust meansfor forcing the first and second clutch plates towards one another andagainst the rolling elements of the bearing, wherein the first andsecond clutch plates are made of an elastically-deformable plasticsmaterial, and wherein the drive member is rotatably mounted on a shaftsleeve by means of an angular contact bearing within the sprocket, thefirst and second clutch plates being generally annular and surroundingthe shaft sleeve, and the shaft sleeve being fixed to, and rotatablewith, the driven member, and the arrangement is such that the rollingelements elastically deform the surfaces of the first and second clutchplates whereby torque can be transmitted from the first clutch plate tothe second clutch plate.
 2. A clutch assembly for an accumulating rollerconveyor, the clutch assembly comprising a drive member formed by asprocket which is drivably engageable with a conveyor drive means, adriven member which is drivably engageable with a roller of theconveyor, a first clutch plate associated with the drive member, asecond clutch plate associated with the driven member, a bearingpositioned between the first and second clutch plates and having arolling element race, and thrust means for forcing the first and secondclutch plates towards one another and against the rolling elements ofthe bearing, wherein the first and second clutch plates are made of anelastically-deformable plastics material, and wherein the drive memberis rotatably mounted on a shaft sleeve by means of an angular contactbearing, the first and second clutch plates being generally annular andsurrounding the shaft sleeve, and the shaft sleeve being fixed to, androtatable with, the driven member, and the arrangement is such that therolling elements elastically deform the surfaces of the first and secondclutch plates to form depressions therein, whereby torque can betransmitted from the first clutch plate to the second clutch plate,wherein the arrangement is such that the torque transmission is amaximum as the clutch begins to slip, the torque transmitted reducing asclutch slip continues and the depressions in said surfaces become lesspronounced.
 3. A clutch assembly as claimed in claim 1 or claim 2,wherein the arrangement is such that the rolling elements of the angularcontact bearing elastically deform the races associated therewith.
 4. Aclutch assembly for an accumulating roller conveyor, the clutch assemblycomprising a drive member formed by a sprocket which is drivablyengageable with a conveyor drive means, a driven member which isdrivably engageable with a roller of the conveyor, a first clutch plateassociated with the drive member, a second clutch plate associated withthe driven member, a bearing positioned between the first and secondclutch plates and having a rolling element race, and thrust means forforcing the first and second clutch plates towards one another andagainst the rolling elements of the bearing, wherein the first andsecond clutch plates are made of an elastically-deformable plasticsmaterial, and wherein the drive member is rotatably mounted on a shaftsleeve by means of another bearing within the sprocket, the first andsecond clutch plates being generally annular and surrounding the shaftsleeve, and the shaft sleeve being fixed to, and rotatable with, thedriven member, and the arrangement is such that the rolling elementselastically deform the surfaces of the first and second clutch plates toform depressions therein, whereby torque can be transmitted from thefirst clutch plate to the second clutch plate.
 5. The clutch assembly ofclaim 4, wherein said another bearing constitutes means for the takingup both radial and longitudinal forces.
 6. A clutch assembly of claim 1or 5, wherein the sprocket is drivably engageable with an endless drivechain constituting the conveyor drive means.
 7. A clutch assembly ofclaim 1 or 5, wherein the first clutch plate is constituted by a surfaceof the sprocket.
 8. A clutch assembly of claim 1 or 5, wherein thethrust means is a thrust nut which is engageable with anexternally-threaded portion of the shaft sleeve.
 9. A clutch assembly ofclaim 1 or 5, wherein the sprocket is made of a low-friction,thermoplastic material such as acetal.
 10. A clutch assembly of claim 1or 5, wherein the driven member is a drive shaft fixed to, and rotatablewith, the roller.
 11. A clutch assembly as claimed in claim 10, whereinthe second clutch plate is constituted by a surface of a retaining ring,the retaining ring being positioned between the thrust means and thebearing.
 12. A clutch assembly as claimed in claim 11, wherein theretaining ring is made of a low-friction, thermoplastic material such asacetal.
 13. A clutch assembly of claim 1 or 5, wherein the shaft sleeveis made of a low-friction, thermoplastic material such as acetal.
 14. Aclutch assembly of claim 1 or 5, wherein the rolling elements of thebearing are balls made of a self-cleaning, high chromium special steel.15. A clutch assembly as claimed in claim 1, wherein said clutchassembly is between said roller and conveyer drive means.
 16. A clutchassembly as claimed in claim 15, wherein the conveyer drive means is anendless chain which engages the sprocket of each clutch assembly of aplurality of clutch assemblies.